1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine. Specifically, the present invention relates to a system having multiple estimation devices that estimate a particulate matter accumulation quantity during a regeneration period of a particulate filter.
2. Description of Related Art
Improvement of exhaust emission of an internal combustion engine of an automobile or the like has been required. Specifically, a diesel engine that uses diesel oil as fuel has to eliminate exhaust particulate matters such as soot in addition to gaseous components such as carbon monoxide, hydrocarbon or nitrogen oxides contained in exhaust gas. Therefore, a particulate filter is located in an exhaust passage to collect the exhaust particulate matters.
If the exhaust gas entering the particulate filter flows downstream and passes through porous partition walls of the particulate filter, the exhaust particulate matters are collected at wall surfaces or pores of the partition walls of the particulate filter. If a quantity of the collected exhaust particulate matters increases excessively, a gas flow resistance at the particulate filter will increase. Accordingly, a back pressure of the engine will increase and an engine output will decrease. Therefore, it is necessary to combust and eliminate the exhaust particulate matters collected in the particulate filter at suitable timing to regenerate an ability of the particulate filter to collect the exhaust particulate matters.
Usually, an accumulation quantity of the exhaust particulate matters (quantity of the exhaust particulate matters accumulated in the particulate filter) is estimated, and the regeneration of the particulate filter is performed when the estimated accumulation quantity reaches a predetermined value. A method (first estimation method) estimates the accumulation quantity of the exhaust particulate matters accumulated in the particulate filter based on a gas-flowing state in the particulate filter. The first estimation method is based on the fact that the gas flow resistance increases as the accumulation quantity of the exhaust particulate matters increases. A pressure difference sensor senses a differential pressure between an inlet and an outlet of the particulate filter, and the method estimates the accumulation quantity based on the sensed value.
Another method (second estimation method) estimates the accumulation quantity based on an engine operation history. The second estimation method senses operating states such as rotation speed or output torque of the engine. The method calculates a generation quantity of the exhaust particulate matters generated by an engine main body, and integrates the generation quantity to estimate the accumulation quantity of the exhaust particulate matters. The second method beforehand prepares a map representing a relationship between the operating state of the engine and the generation quantity of the exhaust particulate matters based on data obtained through bench experimentation and the like.
An exhaust gas purification system described in JP-A-2003-83035 constantly performs both of the accumulation quantity estimation based on the first estimation method and the accumulation quantity estimation based on the second estimation method. If either one of the estimated accumulation quantities exceeds a set value, the system operates temperature increasing device to perform compulsory regeneration of the particulate filter. By using the two estimation methods at the same time, even if a trouble is caused or an error increases in either one of the two methods, the compulsory regeneration can be suitably performed based on the other estimation method to prevent excessive increase of the accumulation quantity.
The first estimation method can perform the estimation accurately because the first estimation method directly senses a degree of clogging. However, if a flow rate of the exhaust gas passing through the particulate filter decreases, the differential pressure will decrease and sensing accuracy of the accumulation quantity will decrease. In a transitional state, a stable differential pressure cannot be obtained, and the accumulation quantity of the exhaust particulate matters cannot be necessarily estimated at sufficient accuracy.
The second estimation method does not directly sense the state of the particulate filter but regards the quantity of the exhaust particulate matters discharged from the engine as the accumulation quantity in the particulate filter, and the second estimation method integrates an anticipated value of the accumulation quantity. This type of estimation contains a variation in the discharge quantity due to a variation in an engine individual body, a change in the operating state (constant speed drive and acceleration/deceleration drive), a change with time (change in characteristics after long-term drive) or environmental changes (temperature, air pressure and the like) as errors. Therefore, if the second estimation method continues for a long time, there is a possibility that the error will accumulate and the estimation accuracy of the accumulation quantity will be deteriorated.
The system described in JP-A-2003-83035 starts the regeneration operation if the estimate of the accumulation quantity of the first or second estimation methods exceeds the same set value. However, in the case where the regeneration operation is started based on the estimate of the second estimation method, the estimation accuracy is relatively low. Therefore, in such a case, there is a possibility that the actual accumulation quantity is too large (actual quantity>estimate). If the particulate filter is regenerated in a state in which the actual accumulation quantity is too large, there is a possibility that the temperature of the particulate filter will increase abnormally and the particulate filter will be damaged due to heat generation caused by combustion of the exhaust particulate matters. If the regeneration operation is performed frequently to avoid the excessive accumulation of the particulate matters, a fuel consumption will be deteriorated.